1. Field of the Invention
The present invention relates to a zoom lens system for an electronic still camera (digital camera) having a wide angle-of-view (i.e., a shorter focal length at the short focal length extremity) and a zoom ratio of approximately 3.
2. Description of the Prior Art
In recent years, a digital camera is not only used as a camera per se, but also provided in a portable data handset (PDA: Personal Digital Assistant) and a mobile phone, etc. An imaging device (imaging module), such as a CCD and a CMOS, used for such a digital camera has been required to be very much miniaturized. Therefore an imaging device with a relatively smaller effective imaging area has often been used; and there have been many digital cameras whose optical system is constituted by a smaller number of lens elements.
As a small-sized zoom lens system which has a smaller number of lens elements, a negative-lead type lens system has been frequently employed. In such a lens system, there are at least two advantages, i.e., (i) the focal length of the lens system can be made shorter at the short focal length extremity, and (ii) telecentricity on the image side can be maintained.
For example, the following Japanese Unexamined Patent Publications (hereinafter, JUPP) have disclosed a negative-lead type lens system of three (and two)-lens-group arrangement, and drawbacks thereof will be discussed hereinafter:
JUPP No.2002-55278, JUPP No.Hei-11-237549, and JUPP No.Hei-10-513270.
In JUPP No. 2002-55278, a zoom lens system of a three-lens-group arrangement has been disclosed, i.e., a first lens group having a negative refractive power (hereinafter, a negative first lens group), a second lens group having a positive refractive power (hereinafter, a positive second lens group), and a third lens group having a positive refractive power (hereinafter, a positive third lens group), in this order from the object; and the third lens group has a weaker positive refractive power (hereinafter, positive power). However, zooming is mainly performed by the first lens group and the second lens group. As a result, the third lens group does not substantially contribute to zooming, so that the zoom lens system has not sufficiently been miniaturized.
In JUPP No. Hei-11-237549, a zoom lens system of a three-lens-group arrangement has been disclosed, i.e., a negative first lens group, a positive second lens group, and a positive third lens group, in this order from the object; and the power of each lens group is weak, and the power of the second lens group is particularly weak. Consequently, the distance between the first lens group and the second lens group becomes longer, so that the overall length of the zoom lens system becomes longer.
In JUPP No.Hei-10-513270, a zoom lens system of a three-lens-group arrangement has been disclosed, i.e., a negative first lens group, a positive second lens group, and a positive third lens group, in this order from the object; and the power of the positive second lens group is weak, so that the zoom ration is less than two. Consequently, this publication is not worth discussing on whether the entire optical system is longer or shorter.
If an attempt is made to make the overall length of the zoom lens system shorter, it is understood that the power of each lens group is made stronger so that the traveling distances of the lens groups become shorter. However, if the power of a lens group is made stronger, aberrations become larger. Consequently, it becomes difficult to correct aberrations suitably over the entire zooming range.